(168b) RHEED-TRAXS Development for Real Time, in-Situ Stoichiometry Analysis During Thin Film Deposition

Multifunctional heterostructures of functional oxides, such as ferroelectric barium titanate and piezoelectric lead zirconate titanate, integrated on semiconductor platforms are of interest to the development of next-generation smarter, smaller and more energy efficient devices. As these multi-element oxide materials exhibit measurably different functional properties with structure changes or stoichiometry changes of less than one percent[1], Molecular Beam Epitaxy (MBE) is used to take advantage of this sensitivity of functional properties by controlling the stoichiometry and structure of each layer. In most multi-element oxide material systems, the same crystal structure can represent a range of relative stoichiometries. Thus, an accurate real time stoichiometry control technique is needed. Reflection High Energy Electron Diffraction - Total Reflection Angle X-ray Spectroscopy (RHEED-TRAXS) has the potential to achieve real time stoichiometry control and explain the growth mechanism in the ultra-high vacuum (UHV) environment of MBE. When incident RHEED electrons with energy in the range of 12-20 keV hit the sample surface, characteristic x-rays fluorescence representative of the film surface stoichiometry is emitted. By measuring the x-ray at or close to its total reflection angle RHEED-TRAXS is reported to probe only the top 20Å of group V elements[2]. While the proof of concept for RHEED-TRAXS has been demonstrated through qualitative analysis of grown films [3] and recently in growing films [4], thorough quantification studies of RHEED-TRAXS systems are not seen in literature. It is unclear as to the relative impact of various MBE system parameters, including geometry, on x-ray signal and hence RHEED-TRAXS spectra.

This work demonstrates RHEED-TRAX as a real-time surface chemistry characterization tool in ultra-high and high vacuum processing environments. The impact of various factors in the MBE system and RHEED-TRAXS collection chamber such as geometry of the optical path, aperture size and emission current of the RHEED electron beam has been identified and evaluated. The Mg Kα line and Si Kα line signal intensity changes are followed using RHEED-TRAXS during the deposition process of magnesium oxide (MgO) films on silicon carbide (SiC) substrates. Both the Si signal attenuation and the Mg signal intensity time profile are used in conjunction with X-ray Photoelectron Spectroscopy (XPS) to study the growth process with varied growth conditions. XPS is also used as a reference to quantify the RHEED-TRAXS spectra. RHEED electron beam with 12.5keV kinetic energy and 7.0μA emission current, along with 30 seconds acquisition time are used to collect spectra for quantification during film deposition.

References: 1. Kanno, I., et al., Crystallographic characterization of epitaxial Pb(Zr,Ti)O3 films with different Zr/Ti ratio grown by radio-frequency-magnetron sputtering. Journal of Applied Physics, 2003. 93(7): p. 4091-4096.

2.Braun, W. and K.H. Ploog, Real-time surface composition and roughness analysis in MBE using RHEED-induced X-ray fluorescence. Journal of Crystal Growth, 2003. 251(1-4): p. 68-72.

3 Shigetomi, J., et al., Initial growth stage of indium arsenide/gallium arsenide studied by RHEED-TRAXS method. Journal of Crystal Growth, 1991. 111(1-4): p. 110-14.

4 Vanmil, B., Gallium nitride growth by RF-plasma assisted molecular beam epitaxy: Determination of surface stoichiometry by RHEED-TRAXS, annealing of gallium nitride:beryllium and the effects of active nitrogen species, surface polarity, and excess gallium-overpressure on high temperature limits. 2005. p. 110 pp.